Phosphates 



time requiring a large quantity of it, and should 

 therefore receive it in a suificiently assimilable form, 

 generally give very good results with slag, especially 

 in slightly acid ground. 



Obviously all the assimilable phosphate is not 

 in a tetracalcic form. It is probably present also 

 in combination with silicate, in a form of double 

 salt. 



Another portion of the phosphoric acid is in- 

 soluble, that is, soluble only in strong acids — and 

 sometimes very inassimilable. 



The value of basic slag depends very largely, 

 therefore, on the proportion of assimilable or in- 

 assimilable phosphates, and it should never be bought 

 on the total content of phosphoric acid alone. 

 When the phosphoric acid is only soluble in strong 

 acids, it should be regarded as inassimilable, or 

 so slowly assimilable that it ought not to be reckoned 

 in the price of the manure. Only solubility in 

 citric acid should be accepted as a basis on which to 

 determine the price. 



Again, the value of slag is in direct relation 

 to its fineness. Phosphate, even though soluble 

 in citric acid, is not easily assimilated in coarse 

 slag, and may not be assimilable at all, seeing the 

 extreme hardness of the particles. But we have 

 explained the desirability of fineness in manures in 

 a preceding chapter. Here it is sufficient to say 

 that the degree of fineness will be another factor in 

 deciding the purchasing value. 



The lime present in slag also plays a very im- 

 portant part. In neutral soils or slightly acid soils 

 which are deficient in lime, it supplies the foundation 



62 



